Jaroslav Flegr beleives that he has been infected by a parasite that is manipulating his behaviour

Parasites passed from cats could be causing schizophrenia in their owners, a scientist has claimed. Studies have also shown behavioral changes in humans, including lower reaction times and a sixfold increased risk of traffic accidents among infected, RhD-negative males, as well as links to schizophrenia including hallucinations and reckless behavior.

Jaroslav Flegr says has come to the conclusion because he himself believes he is a living example.

He says he has been infected by the parasite and it has altered his behaviour over a period of time.

The parasite, which is excreted by cats in their feces, is called Toxoplasma gondii and is the microbe that causes toxoplasmosis – the reason pregnant women are told to avoid cats’ litter boxes.

Since the 1920s, doctors have recognised that a woman who becomes infected during pregnancy can transmit the disease to the foetus, resulting in severe brain damage in the baby – or even death.

In adults the diesease causes flu-like symptoms – and those with a suppressed immune system can become seriously ill with complications such as encephalitis (inflammation of the brain) – but many carrying the latent disease and appear to have no symptoms.

However, once inside an animal or human host, the parasite then needs to get back into the cat, as that is the only place where it can sexually reproduce.

And this, Flegr believes, is why the infection is making subtle changes to the human brain to manipulate the host’s behavior – though he admits to being unsure how the parasite intends to use humans to this end.

But from his own experience the Czech scientist, 63, claims that over the past two decades his personality has been changing, leading him to behave in strange, often self-destructive ways.

Flegr recently told the magazine how he started thinking about his own theory almost 30 years ago after reading a book by the British evolutionary biologist Richard Dawkins.

The Toxoplasma gondii parasite is only able to reproduce in a cat’s gut

The book described how flatworms, to help spread the species, are able to take over an ant’s body by infecting their nervous system.

This then alters the ant’s behaviour such that a drop in temperature that would normally force the insect underground, sees it instead climb to the top of a blade of grass and clamp down on it.

It is then eaten by grazing sheep and the flatworm’s life cycle continues in the animal’s gut.

‘It was the first I learned about this kind of manipulation, so it made a big impression on me,’ Flegr says.

Flegr says he then began to notice similarities between his behaviour and that of the reckless ant.

For example, he says, he thought nothing of crossing a busy road, ‘and if cars honked at me, I didn’t jump out of the way’, he told The Atlantic.

He says he also refused to hide his contempt for the Communists who ruled Czechoslovakia for most of his early adulthood.

‘It was very risky to openly speak your mind at that time,’ he says. ‘I was lucky I wasn’t imprisoned.’

And during time he spent in eastern Turkey, he says he was able to stay ‘very calm’ during gunfire, while ‘my colleagues were terrified. I wondered what was wrong with myself’.

He believes that large numbers of humans could be similarly afflicted with toxoplasmosis, and that it explains reckless behaviour in many humans.

He says those infected with the parasite are, for example, at greater risk of traffic accidents.

His claims come after a study at Leeds University showed the parasite affects the production of dopamine – the chemical that carries messages in the brain controlling aspects of movement, cognition and behaviour – thus triggering schizophrenia and other bipolar disorders.

The parasite infects the brain by forming a cyst within its cells and produces an enzyme called tyrosine hydroxylase, which is needed to make dopamine.

Dopamine’s role in mood, sociability, attention, motivation and sleep patterns are well documented. Dopamine is the target of all schizophrenia drugs on the market.

Individuals with schizophrenia are also more likely to report a clinical history of toxoplasmosis than those in the general population, other studies have shown.

Dr Glenn McConkey, lead researcher on the Leeds University project: ‘It’s highly unlikely that we will find one definitive trigger for schizophrenia as there are many factors involved, but our studies will provide a clue to how toxoplasmosis infection – which is more common than you might think – can impact on the development of the condition in some individuals.

‘In addition, the ability of the parasite to make dopamine implies a potential link with other neurological conditions such as Parkinson’s Disease, Tourette’s syndrome and attention deficit disorders,’ said Dr McConkey.

‘We’d like to extend our research to look at this possibility more closely.’

It is unclear why the parasite has chosen humans as a host. The effect on rodents, however, is more obvious.

In studies, infected rodents were much more active in running wheels than uninfected ones, suggesting they would be more attractive targets for cats, which are drawn to fast-moving objects.

They also were less wary of predators in exposed spaces, Flegr says.

Thus rodents are more easily caught by cats and the parasite can continue its life cycle.

Similar findings suggested the parasite makes rats less scared of – and indeed almost attracted to – cats.

Joanne Webster, a parasitologist at Imperial College London, also suggest rats would be likely targets for behavioral manipulation from a a parasite that can only thrive in cats

She found, as previous researchers had shown, that infected rats were more active and less cautious in areas where predators lurk.

But then, in another experiment she treated one corner of each rat’s enclosure with the animal’s own odor, a second with water, a third with cat urine, and the last corner with the urine of a rabbit, a creature that does not prey on rodents.

‘We thought the parasite might reduce the rats’ aversion to cat odour,’ she told me. ‘Not only did it do that, but it actually increased their attraction. They spent more time in the cat-treated areas.’

Separate studies have linked the parasite to increased rates of suicide, according to Teodor Postolache, a psychiatrist and the director of the Mood and Anxiety Program at the University of Maryland School of Medicine.

He looked at a raft of studies that included investigations of general populations as well as groups made up of patients with bipolar disorder, severe depression, and schizophrenia, and in places as diverse as Turkey, Germany, and the Baltimore/Washington area in the U.S.

Though exactly how the parasite may push vulnerable people over the edge is yet to be determined.